Investigation of the local environment of Eu3+ in a silicophosphate glass using site-selective spectroscopy and Molecular Dynamics simulations

Silicophosphate glasses (SiO2-P2O5) doped with Eu3+ ions were synthesized by the sol-gel process. Optical properties of these glasses were investigated by means of emission spectra and lifetime measurements. The Fluorescence Line Narrowing (FLN) technique was also used to explore the local structure around the Eu3+ ions in this host and to understand the role of phosphate as a codopant. As it is the case for aluminum, the ability of phosphate to avoid the rare earth clustering was investigated, and the role of this codopant in modifying the local order around the rare earth ion was evidenced. The analysis of the FLN spectra and lifetime measurements is consistent with this interpretation. Molecular Dynamics simulations were performed to evaluate and confirm these structural features. Two classes of europium sites were distinguished in agreement with the experimental characterization

Fluctuations in active membranes

Active contributions to fluctuations are a direct consequence of metabolic energy consumption in living cells. Such metabolic processes continuously create active forces, which deform the membrane to control motility, proliferation as well as homeostasis. Membrane fluctuations contain therefore valuable information on the nature of active forces, but classical analysis of membrane fluctuations has been primarily centered on purely thermal driving. This chapter provides an overview of relevant experimental and theoretical approaches to measure, analyze and model active membrane fluctuations. In the focus of the discussion remains the intrinsic problem that the sole fluctuation analysis may not be sufficient to separate active from thermal contributions, since the presence of activity may modify membrane mechanical properties themselves. By combining independent measurements of spontaneous fluctuations and mechanical response, it is possible to directly quantify time and energy-scales of the active contributions, allowing for a refinement of current theoretical descriptions of active membranes.Comment: 38 pages, 9 figures, book chapte

Molecular dynamics study of rare earth-doped Mg-silicate nanoparticles in vitreous silica: from the preform to the fiber

International audienceA Molecular Dynamics study of rare-earth doped Mg-silicate nanoparticles in vitreous silica: from the preform to the fiber. New lasers and amplifiers still require an enhancement of the spectroscopic performance of rare-earth-doped silica optical fibers. In order to tailor their optical behavior, a route of interest consists in embedding rare-earth ions within dielectric nanoparticles in the core of optical fibers. Nanoparticles are formed through spontaneous phase separation phenomenon within a MgO–SiO2 binary melt, during melt/quench sequences of MCVD fabrication process of the preform [1][2]. Then, fibers are obtained by drawing at high temperature a preform containing nanoparticles. First report on the drawing process reveals an elongation of the nanoparticles in the drawing direction as well as a breakup of the larger ones [3]. In this Molecular dynamics study, we use a new simple transferable model [4] to show that phase separation occurring in the MgO–SiO2 binary melt, leads to separation of liquid phases with mixed composition: Si-rich Mg-poor phases on one hand, Mg-rich Si-poor phases on the other hand. These latter phases, the so-called nanoparticles, are amorphous, non spherical and exhibit a wide range of sizes. Mg-O coordination and MgO content increase with the nanoparticle size. With rare-earth doping, the larger nanoparticles are over-concentrated in luminescent ions. Due to an oxygen-rich environment in the nanoparticles, we show that the rare-earth clustering effect is greatly prevented, compared with a pure silica matrix. Finally, at high temperature, we apply a uniaxial elongation to the nanostructured preform to mimic the experimental drawing step leading to the fiber. We report here results on the effects of this drawing process on the nanoparticles characteristics

Tm-doped nanoparticles in optical fibers

International audienceThe success of silica-based optical fibers are many: transmission fibers and fiber amplifiers for telecommunications, high-power fiber lasers or sensors. These key applications rely on the qualities of silica glass: mechanical and chemical stability, high optical damage threshold, low cost, etc. New lasers and amplifiers based on rare-earth (RE)-doped silica optical fibers need improved spectroscopic performances : gain curve engineering, photodarkening, spectral coverage, etc. In this context, a route of interest consists of embedding the RE ions within nanoparticles of composition and structure different from those of silica. In this work, we study the properties of silica-based, MCVD-prepared fibers using LaF3:Tm3+ nanoparticles. The nanoparticles with 10-20 nm of diameter were produced by precipitation methods and were incorporated by solution doping. Through SEM analyses on preform and fiber, nanoparticles were observed across the core. As F-ions evaporate, the new phase is a La-rich silicate and its composition will be discussed based on the comparison with similar samples prepared by sol-gel. The first e-folding time of the 810-nm emission band (3H4 level) increases with the concentration of La. The best compromise between lifetime enhancement and optical attenuation corresponds to 58 μs and 0.05 dB/m, respectively. Yet, to further improve the optical properties, there is a need to limit Rayleigh scattering induced by the presence of nanoparticles. In the frame of these optical losses reductions, we propose to take advantage of the fiber drawing to tailor the size of nanoparticles. Indeed, we will report evidences that this step permits the deformation and break-up of elongated particles. The possibility of considering break-up as a way to implement size tailoring of nanoparticles will be discussed. These results clearly offer new possibilities for the control of the luminescent properties and the development of optical fibers with augmented properties

On the morphologies of oxides particles in optical fibers: Effect of the drawing tension and composition

International audienceRare-earth-doped oxide nanoparticles in the core of silica optical fibers are becoming well studied as they yield enhanced and tailorable spectroscopic and optical properties. In this paper, the evolution of particle morphology , induced by the drawing step, is studied. Indeed, during the fiber draw process, the glass flows and particles can elongate and even break-up into smaller particles through Rayleigh-Plateau instabilities. The shape of elongated particles is related to the composition as it depends on the viscosity ratio between the particle and the matrix. Moreover, a lower drawing temperature enhances the break-up phenomenon. These observations offer new possibilities for the control of the size and the shape of particles, hence performance of active optical fibers